Toxoplasma gondii is an obligate intracellular parasite and an important opportunistic pathogen in AIDS patients. Current drug treatments do not target the chronic life stage and are poorly tolerated and therefore new drug targets are constantly needed. Toxoplasma lives inside a parasitophorous vacuole (PV), which is surrounded by the PV membrane (PVM). Molecules, such as certain host nutrients, smaller than 1300-1900 Da can freely diffuse into the PV presumably through pores in the PVM. The molecular basis for these pores is currently unknown. We have identified a Toxoplasma protein, GRA17, which is present on the PVM and influences the parasite's access to small molecules from the host. GRA17 is essential for Toxoplasma growth in vitro and virulence in vivo. We hypothesize that GRA17 is the Toxoplasma PVM pore or influences its activity. Because such a pore would be an excellent drug target we propose to further characterize GRA17 and its role in host nutrient import. To do this we will first precisely define the physical parameters of the Toxoplasma PVM permeability to small molecules mediated by GRA17. We will test whether GRA17 directly alters the permeability of membranes by expressing it in a heterologous system, Xenopus laevis oocytes, where we will measure its activity electro physiologically. We will also attempt to identify the relevant PVM-fluxed biomolecules whose transport GRA17 affects. Second, we will investigate by what mechanism GRA17 is able to affect the permeability of the PVM to small molecules. We will carry out a structure-function based approach to define the relevant functional motif(s) of GRA17, which has no such already defined motifs besides a predicted predominately ?elical structure. Finally, we will characterize a putative GRA17-physical interactor we have already found and will continue searching for other physical interactors through alternate means.